Submitted to: Precision Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2005
Publication Date: 7/1/2006
Citation: Fitzgerald, G.J., Rodriguez, D., Christensen, L.K., Belford, R., Sadras, V.O., Clarke, T.R. 2006. Spectral and thermal sensing for nitrogen and water status in rainfed and irrigated wheat environments. Precision Agriculture. p. 1-16. Interpretive Summary: Farmers who have adopted site-specific management practices (managing units smaller than the boundaries of a given field) need within-field information about crop conditions. Crop status with respect to water and nitrogen conditions is especially important for managing inputs such as water and fertilizer. Water-deficient plants are less able to take up nitrogen for growth than plants with sufficient water. Additionally, knowing which plants are nitrogen-deficient could allow targeted applications of nitrogen. Thus, if areas in a field could be identified under water-stressed conditions, nitrogen status would need to be ascertained only in those areas of a field where plants could utilize additional nitrogen fertilizer inputs. This paper presents remotely sensed measures that can detect water stress and nitrogen status and could potentially direct a producer to apply nitrogen only to those parts of a field not undergoing water stress and that require nitrogen. This could reduce input costs and environmental consequences from excess nitrogen losses from the fields. The results of this study will benefit producers and consultants who are implementing site-specific management, as well as other scientists studying the application of remotely sensed data to agricultural and environmental issues.
Technical Abstract: Variable-rate technologies and site-specific crop nutrient management require real-time spatial information about the potential for response to in-crop management interventions. Thermal and spectral properties of canopies can provide relevant information for non-destructive measurement of crop water and nitrogen stress. In previous studies, foliage temperature was successfully estimated from canopy-scale (mixed foliage and soil) temperatures, and the multispectral Canopy Chlorophyll Content Index (CCCI) was effective in measuring canopy-scale N status in rainfed wheat (Triticum aestivum L.) systems in Horsham, Victoria, Australia. In the present study, results showed that under irrigated wheat systems in Maricopa, Arizona, U.S.A., the theoretical derivation of foliage temperature unmixing produced relationships similar to those in Horsham. Derivation of the CCCI led to an r2 relationship with chlorophyll a of 0.53 after Zadoks stage 43. This was later than the relationship (r2=0.68) developed for Horsham after Zadoks stage 33 but early enough to be used for potential mid-season N fertilizer recommendations. Additionally, ground-based hyperspectral data estimated plant N (g kg-1) in Horsham with an r2=0.86 but was confounded by water supply and N interactions. By combining canopy thermal and spectral properties, varying water and N status can potentially be identified eventually permitting targeted N applications to those parts of a field where N can be used most efficiently by the crop.